The present invention relates to a straight steel section excellent in joint fatigue property. More particularly, the invention relates to a straight steel section used for connecting component members used for forming a civil engineering structure, and among others, applied to members required to have a satisfactory joint fatigue property, and a manufacturing method thereof.
A straight steel section has, as shown in FIG. 1, a joint 2 comprising a curved claw 20 and a ball claw 21 at the both ends of a straight web 1. The bag-shaped space surrounded by the curved claw 20 and the ball claw 21 is called a joint pocket 22, and the exit thereof is called a joint opening 23. When connecting two straight steel sections together to form a joint, the ball claw 21 of a straight steel section is inserted into the joint pocket 22 of the other straight steel section.
Rolling (hot rolling) favorable for productivity, particularly caliber rolling using caliber rolls is commonly adopted for manufacturing straight steel sections.
FIG. 2 is a caliber system diagram illustrating a typical caliber rolling process of a straight steel section. As shown in FIG. 2, the straight steel section is usually manufactured through a first step for preparing a section blank having flanges 2A at the both ends of the web 1 by rolling vertically symmetrically a bloom through, for example, calibers K14 to K11, a second step for rolling vertically asymmetrically the section blank through, for example, calibers K10 to K3 to adjust dimensions (width and thickness) of the web 1 and forming the flange A into a rough joint 2B having a projection 20A and a ball claw 21, and a third step for finishing the rough joint 2B into a joint 2 by forming a curved claw 20 by pressing and bending the projection 20A onto the anti-web side through, for example, calibers K2 and K1 (this is called the xe2x80x9cclaw bendingxe2x80x9d).
FIG. 3 is a layout drawing illustrating a typical caliber rolling equipment corresponding to FIG. 2. In this example, the calibers K14 to K11 are allocated to a blooming mill (BM mill); the calibers K10 to K7, to a breakdown mill (BD mill); the calibers K6 to K4, to an intermediate mill (S1 mill); and the calibers K3 to K1, to a finishing mill (SF mill). The section blank manufactured in the first step is usually left to cool to near room temperature, then reheated and subjected to hot rolling in the second and subsequent steps.
The claw bending process through the calibers K2 and K1 is illustrated in FIG. 4. As shown in FIG. 4, claw bending is conducted through changes in the gap between the upper and lower rolls along with the progress of rolling. In FIG. 4, the reference numeral 20B represents a bent portion during deformation from the projection 20A into the curved claw 20.
The straight steel section manufactured by this process gives a very high productivity and is mass-producible as compared with a straight steel section manufactured by the hot extrusion forming process, and thus provides a remarkable merit of stable supply at a low cost.
For the purpose of smoothing traffic for alleviation of traffic jam in cities, overhead crossing of railroads with roads is now promoted. Grade separation of crossing includes an underpass in which a road passes under a railroad and an overpass in which the road passes over the railroad. With a view to reducing the construction period and the cost in the underpass process, a process using straight steel sections (JES (Jointed Element Structure) process) is attracting the general attention. Details of this process are shown in FIG. 5. This is a tunnel wall building process for installing a new road tunnel 30 under a railroad 60. In this process, asymmetric connecting elements 400, each comprising two asymmetric connecting element members 4 and a connecting plate 41 welded together in staple shape, are sequentially connected through engagement of joints 40 and 40, thus permitting easy construction of a structure 300 (the tunnel wall frame, in this case), not requiring separate preparation of construction scaffold. It is attracting the general attention as a process favorable in period and cost aspects. The asymmetric connecting element member 4 can be manufactured by cutting the straight steel section in FIG. 1 at the width center of the web 1 thereof, turning one of the cut portions upside down, and welding a separately prepared flat plate in between.
When manufacturing a straight steel section, as described above, a curved claw is formed in the third step of the caliber rolling process. Upon bending the claw, wrinkle flaws 10 are formed on the inner surface of the curved claw 20 as shown in FIG. 6.
Such wrinkle flaws have never posed a problem. More specifically, a straight steel section has usually a relatively small joint thickness as up to about 16 mm (for the evaluated site, see FIG. 1). Produced wrinkle flaws have as well a small depth, and this sufficiently ensures a required static tensile strength. This is why wrinkle flaws have not been considered to pose any problem.
However, as is suggested by the structural element member in the aforementioned JES process, there is a tendency toward requiring a higher joint strength. To meet such a demand, it is necessary to use a joint thickness larger than the conventional one. In this case, there occurs a larger contraction of the inner surface of the curved claw upon claw bending, leading to an increase in the wrinkle flaw depth. When applied to a structural member in which a cyclic stress acts on the joint, the wrinkle flaws present on the claw inner surface exert a notch effect, and this results in a problem of deterioration of the fatigue life. That is, at every passage of a train on the rail, the load thereof repeatedly acts particularly on the upper slab of the railroad, so that engagements of joints 40 of asymmetric connecting element member, among others, are susceptible to fatigue. As a result, a straight steel section used for this purpose is required to be excellent in fatigue property in the joint, particularly in the curved claw. The relationship between wrinkle flaws and fatigue property has not however as yet been clarified.
The present invention has therefore an object to the extent of wrinkle flaws not affecting fatigue property, and to provide a straight steel section which permits reduction of wrinkle flaws produced on the inner surface of the joint and is excellent in joint fatigue property and a manufacturing method thereof.
The present inventors carried out studies for the purpose of improving joint fatigue property of a straight steel section, and found measures to reduce the wrinkle flaw depth on the inner surface of the joint throughout the entire rolling process of straight steel sections, a chemical composition satisfying strength and weldability requirements as a connecting element member, and the relationship with rolling and bending-forming conditions permitting reduction of the wrinkle flaw depth, thus completing the present invention. The gist of the invention is as follows:
(1) A straight steel section excellent in joint fatigue property, having a joint comprising a flat web and a ball claw and a curved claw at the both ends in the width direction thereof, wherein wrinkle flaws present on the inner surface of the curved claw have a depth of 0.5 mm or less.
(2) A straight steel section excellent in joint fatigue property according to (1) above, having a chemical composition comprising, in mass percentage, from 0.01 to 0.20% C, 0.8% or less of Si, 1.8% or less of Mn, 0.030% or less of P, and 0.020% or less of S, and the balance Fe and incidental impurities.
(3) A straight steel section excellent in joint fatigue property according to (1) above, having a chemical composition comprising, in mass percentage, from 0.01 to 0.20% C, 0.8% or less of Si, 1.8% or less of Mn, 0.030% or less of P, and 0.020% or less of S, and in addition, one or more selected from the following groups 1 to 3:
(group 1) one or more selected from the group consisting of 1.0% or less of Cu, 1.0% or less of Ni, 1.0% or less of Cr, 0.5% or less of Mo, 0.10% or less of V, 0.10% or less of Nb, and 0.005% or less of B;
(group 2) 0.1% or less of Al; and
(group 3) one or more selected from the group consisting of 0.10% or less of Ti, 0.010% or less of Ca, and 0.010% or less of REM, and the balance Fe and incidental impurities; wherein the carbon equivalent Ceq defined by the following formula (1):
Ceq=C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/14xe2x80x83xe2x80x83(1) 
where each of element symbols on the right side:
content of the element (mass %);
is 0.45% or less.
(4) A straight steel section excellent in joint fatigue property according to any one of (1) to (3) above, which is used in a tunnel wall frame member for constructing a road under a railroad.
(5) A manufacturing method of a straight steel section excellent in joint fatigue property, having a joint comprising a flat web and a ball claw and a curved claw at the both ends in the width direction thereof, comprising a first step for hot rolling a bloom vertically symmetrically to make a section blank having a flange at a web end, a second step for hot-rolling the section blank vertically asymmetrically to adjust the size of the web and form the flange into a rough joint including a projection, and a third step for finishing the rough joint into a joint by hot-bend-rolling the projection into a curved claw; wherein the bloom has a chemical composition comprising, in mass percentage, from 0.01 to 0.20% C, 0.8% or less of Si, 1.8% or less of Mn, 0.030% or less of P, and 0.020% or less of S; and wherein the claw bending start temperature in the third step is a temperature of over Ar3 or Ar3-50xc2x0 C. or below.
(6) A manufacturing method of a straight steel section excellent in joint fatigue property according to (5) above, wherein the claw bending end temperature in the third step is 700xc2x0 C. or more.
(7) A manufacturing method of a straight steel section according to (5) or (6) above, wherein the flange outer surface of the section blank is smoothed in cold during the interval between the first step and the second step.
(8) A manufacturing method of a straight steel section according to (7) above, wherein the smoothing treatment is carried out so that the smoothed surface has a surface roughness Rmax of 20 xcexcm or less.